Evolution of grain boundary character distributions in FCC and BCC materials

grantor: University of TorontoThe structures of grain boundaries were characterized within the framework of Coincidence Site Lattice (CSL) model using Orientation Imaging Microscopy (OIM$\sp{\rm TM}).$ The Grain boundary character distribution (GBCD) in FCC low stacking fault energy materials susceptible to annealing twinning, showed a preference of $\Sigma 3\sp{\rm n}$ boundaries (where n = 1, 2 and 3). The changes in the distribution of the CSL boundaries during microstructural evolution were found to be influenced by thermomechanical treatments, solute contents, texture and grain size. Moreover, the contribution of twinning to the CSL distribution was further discussed in terms of the 'Twin Limited Microstructure' whereby the frequencies of special grain boundaries $(\Sigma \le 29)$ were found to lie within limits defined by (a) an upper limit based upon twin energetics leading to the creation of additional special grain boundaries, and (b) a lower limit arising from isolated twinning events. The preference for $\Sigma 3\sp{\rm n}$ grain boundaries was attributed to the geometric interaction of twin related variants during grain growth. The role of CSL boundaries in the development of Goss texture in BCC Fe-3% Si alloy was investigated in primary recrystallized specimens and in specimens following the early stages of secondary recrystallization. By utilizing the direct experimental crystallographic orientation determination and the approach of 'Simulation by Hypothetical Nucleus (SH)', it was found that (1) Goss grains do not have the size advantage or occur in clusters in the primary matrix; (2) they have a higher probability of forming low $\Sigma$ CSL boundaries, especially the $\Sigma 5$ coincidence relationships. Furthermore, the experimentally determined 'deficit' of CSL boundaries bounding a growing Goss grain relative to the SH distribution, was rationalized on the basis of preferential replacement of CSL boundaries by general boundaries due to the intrinsic enhanced mobility for low C grain boundaries. Such an interpretation supports the role of CSL boundaries in the development of Goss texture and it is further estimated that the CSLs are on average, 10-20% more mobile than general boundaries during abnormal grain growth in Fe-3% Si alloy.Ph.D